A DI box is used when you want to record an instrument, such as a guitar or a keyboard, without using a microphone to capture an amplifier's output. The device allows you to connect one of these instruments directly into the mixing console in your studio, so it can then be recorded directly.
The DI stands for 'direct injection' or 'direct input'. Connecting an instrument directly into a console results in a clean and bright sound with plenty of presence - it'll sound completely different to the real-world body, grit and distortion of an amplifier.
In the studio, recording in this way is often done at the same time as recording an amplifier with a microphone. Both signals will be recorded at the same time which helps to give you more options at the mixing stage. This is now my own standard way of recording a guitar or a bass - I wouldn't do it any other way.
The two tracks can then be mixed together to give a combination of the two tones. You'll often find that the recording from the amp will be the main signal used and the direct recording from the box will be mixed in at a lower level, just enough to give the overall sound a bit more clarity and presence.
Having a recording of the clean, un-amplified signal also means that you'll have the option of re-amping at a later stage. This is where you can send the previously recorded track out of your computer or console and into an amplifier, and then re-record this new amplified output with a microphone.
This is a great way to play around with different types of mic or amp settings to help achieve the sound you're after. I love re-amping guitar recordings and trying out different effects combinations or settings - the flexibility to do this just by using a DI box should be used all the time, in my opinion.
As well as the variation in tone, a DI box (also known as a DI unit) helps to balance the signal levels of different cable and connection types found in the recording studio:
You don't need to know what all of these technical terms mean, but you can see that the two signals are different and don't match. DI units bridge this gap and convert unbalanced line-level signals to balanced microphone-level signals, which helps with the following:
It results in a much cleaner and smoother recording.
Most boxes found today are known as 'active' boxes. These require a power source, which can be received from an external power cable or internal batteries. Some provide the option of using phantom power directly from a mixing console as well.
You'll usually find other options such as attenuators or pads which can decrease the recording signal for levels such as -20 to -60 dB. They might also include hi-pass and lo-pass filters, as well as ground lift, which can help get rid of ground hum that can ruin a recording.
So as we've seen, a DI box has a few different uses and applications and so is a handy bit of gear to have in the studio. It can help to give you a few more options when it comes to recording guitars, basses and keyboards and I'd highly recommend getting one.